Method for production of a hollow body by extrusion and blowing of a thermoplastic resin

ABSTRACT

A method for production of a hollow body by extrusion and blowing of a thermoplastic resin, includes the steps of filling a quantity of resin into an accumulator ( 1 ) which has an accumulation chamber ( 12 ) with an open end, placing a blowing mould ( 24 ) with an internal cavity ( 36 ) and a constricting section ( 37 ) in communication with the open end, axially displacing a core ( 10, 11 ) across the constriction point of the internal cavity such as to cover a projecting part of the core with a layer of resin ( 38 ), continuing the axial displacement of the core in the internal cavity ( 36 ) to axially extend the layer of resin ( 38 ) and applying a fluid pressure to the internal surface of the resin layer through the core to obtain a hollow body.

The present invention relates to a method of manufacturing a biaxiallyoriented hollow body by extrusion blow molding of a thermoplastic resinand to a device for implementing this method.

Document EP 486 419 describes such a method, which comprises the stepsconsisting in bringing the resin into a malleable state, filling anaccumulator with a quantity of said resin, said accumulator having anaccumulation chamber defined between a central core and an outer wallhaving an end opening, placing a blowing mold with an internal cavityhaving an open constriction in communication with said end opening,axially displacing a moveable mandrel from said central core throughsaid accumulation chamber, said end opening and said constriction of theinternal cavity, so as to coat a portion of the mandrel projecting fromsaid central core with a layer of resin.

In this known method, the resin is then blow-molded. This method provesto be advantageous in terms of productivity and allows the temperatureof the resin to be controlled and made uniform, thus preventing weightand size irregularities in the hollow body obtained. However, this knownmethod is not suitable for the manufacture of hollow bodies of largecapacity, since there is a risk of the parison becoming detached fromthe mandrel during the blowing step if said parison has too great aweight. It is known to use a high-molecular-weight thermoplastic resinin order to prevent the risk of parison detachment. However, such aresin has a high glass transition temperature, requiring greater energyconsumption in the molding machine. In addition, such a resin does notalways have mechanical properties suitable for the application of thehollow body. Finally, this known method makes it possible to manufactureonly hollow bodies of simple structure, it being possible for only theexternal form to be adapted by the choice of shape of the blowing mold.

The object of the present invention is to manufacture a biaxiallyoriented hollow body that remedies these drawbacks.

To do this, the invention provides a method of the abovementioned type,characterized by the steps consisting in:

continuing the axial displacement of the moveable mandrel in saidinternal cavity at least as far as an intermediate level between saidopen constriction and an opposite end wall of said internal cavity,simultaneously pushing the resin out of said accumulation chamber withan output speed that is lower than the displacement speed of themandrel, in order to stretch said layer of resin axially;

applying fluid pressure to said inner surface of the layer of resinthrough said mandrel in order to stretch said layer of resintransversely as far as the walls of said internal cavity and obtain abiaxially oriented hollow body having a neck corresponding to theconstriction of the internal cavity; and

letting said hollow body cool down to a rigid state, retracting saidmandrel and ejecting said hollow body from the blowing mold.

In this method, a layer of resin coats the moveable mandrel at least asfar as an intermediate level between the constriction and the end wallof the mold cavity, so as to form a parison directly in the blowingmold, thereby making it possible to carry out the blowing stepimmediately afterwards, without changing work station. The continuityand speed of execution of these two steps avoids problems in the thermalconditioning of the resin. This method operates with most commerciallyavailable resins such as, for example, PVC, polypropylene PP,polyethylenes PE, PET and polyamides PA. Thus, a biaxially orientedhollow body is obtained without a weld with a wall free ofinhomogeneities or other point defects. The use of a coated mandrelallows a parison of large mass to be produced without any risk ofparison detachment, thereby making it possible to obtain a hollow bodyof large capacity and/or with a large wall thickness. The mandrelfulfills both the parison stretch function and the parison supportfunction.

Preferably, the method according to the invention comprises the stepconsisting in impressing an external relief of the projecting portion ofsaid mandrel onto an inner surface of said layer of resin so as toobtain a hollow body having a corresponding internal relief. Forexample, the projecting portion of the mandrel includes grooves and/orparts having different transverse dimensions and/or a thread.

According to one particular embodiment of the invention, said externalrelief includes at least one threaded mandrel portion for obtaining acorresponding thread on the inner surface of said hollow body. Thethread obtained on the inner surface of the hollow body has theadvantage of providing a pressure-resistant attachment for a plug, avalve or a similar accessory that has to be fitted into the neck of thehollow body.

Advantageously in this case, said moveable mandrel comprises aperipheral sleeve that constitutes said threaded mandrel portion and acentral rod that can slide axially relative to said peripheral sleeve.In the displacement step of the mandrel, said peripheral sleeve isbrought into said constriction of the internal cavity so as to clampsaid layer of resin between said threaded mandrel portion and a wall ofsaid constriction, and, in the retraction step of the mandrel, saidperipheral sleeve is made to undergo an axial rotational movement so asto disengage said peripheral sleeve from the internal thread obtained ina corresponding constriction of the hollow body.

The use of a mandrel made in two portions, namely a central rod and athreaded peripheral sleeve, makes it possible to control the movementsof the threaded mandrel portion independently. The peripheral sleevesurrounds the central core so that the gap between the mandrel and theconstriction of the mold is reduced when the peripheral sleeve isintroduced into the constriction. The layer of resin clamped between thetwo parts forms the neck of the hollow body and is shaped, on its innersurface, to the thread of the peripheral sleeve. The internal thread mayalso be formed in another portion of the hollow body, for example in theend wall using a corresponding relief on the end portion of the centralrod.

Advantageously, the end opening of the accumulator and the constrictionof the blowing mold communicate through an extrusion orifice of anextrusion die and, for example at the end of the displacement step ofthe moveable mandrel, a compacting sleeve is moved around said mandrelin said extrusion orifice, said compacting sleeve being inserted betweensaid mandrel and a wall of said extrusion orifice so as to completelyremove the resin from the extrusion orifice in the internal cavity ofthe blowing mold. Thus, the hollow bodies are obtained with a neckcontaining no sink marks.

Preferably, the method according to the invention furthermore includesthe steps consisting in: displacing the peripheral sleeve from theconstriction toward the interior of the internal cavity during theblowing step, so as to fold a flat of said layer of resin between aportion of said layer of resin pressed against the wall of the internalcavity of the blowing mold and an end portion of said layer of resinfastened to the peripheral sleeve; and pressing said folded flat againstsaid end portion of said layer of resin fastened to the peripheralsleeve at the end of the blowing step. Thus, a double-walled neck isobtained, giving it greater rigidity. This neck is provided with aninternal thread for the attachment of a plug or the like. The pressureresistance of the corresponding assembly is also increased.

Advantageously, in the displacement step of the moveable mandrel, theaccumulation chamber is completely emptied through the extrusionorifice. Complete removal from the accumulator allows precise control ofthe quantity of resin that is molded, for precise dimensional control ofthe walls of the hollow body obtained, at a set maintained temperature.

The axial displacement of the mandrel is carried out according to thedesired degree of stretch. In one particular embodiment of theinvention, the mandrel is displaced substantially as far as the end wallof the internal cavity.

The invention also provides a device for implementing this method, whichcomprises:

a resin accumulator that includes an outer wall and a central core thatdefine, between them, an accumulation chamber capable of receiving athermoplastic resin in a malleable state, an end opening made throughsaid outer wall, an extrusion ram arranged so as to slide between saidouter wall and said central core in order to expel the resin from saidaccumulation chamber through said end opening;

a biaxial-orientation blowing mold with an internal cavity having anopen constriction that can be placed opposite said end opening and anend wall on the opposite side from said open constriction;

a mandrel that can move axially between a retracted position inside saidcentral core and projection positions, in which positions a portion ofsaid mandrel projecting from said central core is engaged through saidend opening and said constriction of the internal cavity, said mandrelhaving an axial internal duct opening onto the outside of said mandrelat said projecting portion and a valve for selectively opening andclosing said internal duct;

controlled drive means for selectively displacing, so as to slideaxially, said extrusion ram and said mandrel; and

a pressure source connected to said internal duct of the mandrel,

characterized in that said mandrel can move in said internal cavity atleast as far as an intermediate level between said constriction and saidend wall.

Advantageously, said mandrel has at least one groove for obtaining a ribof corresponding shape on the inner surface of said hollow body.

According to particular embodiments, said groove or each of said groovesfollows a closed annular path or a substantially linear axial path or ahelical path.

Preferably, said mandrel includes at least one threaded mandrel portionfor obtaining a corresponding thread on the inner surface of said hollowbody.

Advantageously, the moveable mandrel comprises a peripheral sleeve thatconstitutes said threaded mandrel portion and a central rod that canslide axially relative to said peripheral sleeve, and said drive meansare capable of axially displacing said central rod and said peripheralsleeve so as not to be in phase and of rotating at least said peripheralsleeve in the unscrewing direction of the thread of the peripheralsleeve.

According to one particular embodiment of the invention, the central rodis rotated axially by said drive means, a unidirectional coupler beingplaced between said peripheral sleeve and said central rod in order torotationally couple said peripheral sleeve to said central rod in saidunscrewing direction and to rotationally decouple said peripheral sleevefrom said central rod in the opposite direction.

Preferably, the end opening of the accumulator and the constriction ofthe biaxial-orientation blowing mold communicate through an extrusionorifice of an extrusion die, a compacting sleeve being placed aroundsaid mandrel and able to move axially between a retracted position insaid central core of the accumulator and a deployed position, in whichposition said compacting sleeve is inserted between said mandrel and awall of said extrusion orifice so as to completely remove the resin fromthe extrusion orifice in the internal cavity of the biaxial-orientationblowing mold.

Advantageously, the outer wall of the accumulator is provided with aheating means and the central core of the accumulator is provided withan internal circuit intended for the circulation of a heat-transferfluid. With these features, the temperature of the resin in theaccumulator is regulated from both faces of the accumulation chamber.The resin may thus be maintained at a uniform temperature optimal forthe molding. For example, the heating means is an electrical resistor.Any other type of heating means may be provided on or in the outer wallof the accumulator and on or in the central core in order for the resinto be thermally regulated simultaneously from the inner peripheralsurface and the outer peripheral surface of the accumulation chamber.

The invention will be better understood, and other objects, details,features and advantages thereof will become more clearly apparent, overthe course of the following description of several particularembodiments of the invention, given solely by way of illustration butimplying no limitation, with reference to the appended drawings. Inthese drawings:

FIG. 1 is a partial view in axial section of a device according to afirst embodiment of the present invention, the accumulator beingassociated with a molding station;

FIG. 2 is an enlarged detail of part of the accumulator of FIG. 1, theaccumulator being associated with an injection station;

FIG. 3 is a view similar to FIG. 1, showing an extrusion step with acoating of the mandrel;

FIG. 4 is a view similar to FIG. 3, showing a biaxial orientation stepwith preblowing;

FIG. 5 is a view similar to FIG. 4, showing the end of the blowing step;

FIG. 6 is an enlarged detail of a device according to a secondembodiment of the present invention, the accumulator being associatedwith a molding station;

FIG. 7 is a partial view showing an alternative embodiment of themandrel;

FIG. 8 is a diagram showing the timing of the operating steps of thedevice of FIG. 1; and

FIG. 9 shows an example of a hollow body obtained using the device ofFIG. 6.

A molding machine for extrusion blow molding according to the firstembodiment of the invention and its operation will now be described.

Referring to FIG. 1, the machine comprises an accumulator 1 that ismounted on a moveable support in order to be able to be associated withtwo different work stations. In FIG. 1, the accumulator 1 is associatedwith a molding station 2.

The accumulator 1 comprises a tubular outer casing 3 fixed at its upperend to a support flange 4. The support flange 4 forms part of aturntable, known per se but not shown, for moving the accumulator 1 fromone work station to the other. The outer casing 3 has, at its lower end,a transverse rim 5 that surrounds and defines an output opening 6 of theaccumulator 1. Inside the outer casing 3 is a central core 7 formed fromseveral coaxial parts that can move relative to one another, namely aninner jacket 8, a compacting sleeve 9, a threaded sizing sleeve 10 and acentral hollow rod 11. The inner jacket 8 comprises several individualparts that contain a circuit for the circulation of a heat-transferfluid, such as thermal oil. The circuit comprises annular ducts 13 madenear the outer surface of the inner jacket 8. The sizing sleeve 10 andthe central hollow rod 11 constitute a coating mandrel, the function ofwhich will be explained below.

Between the central core 7 and the internal wall of the outer casing 3is an accumulation chamber 12 that extends as far as the output opening6 and comprises an annular chamber closed at its upper end 15 by anextrusion ram 14. In FIG. 1, the extrusion ram 14, the inner jacket 8,the compacting sleeve 9, the sizing sleeve 10 and the central hollow rod11 are shown in a retracted position inside the outer casing 3. Thesevarious parts may be moved axially toward the outside of the outercasing 3 by a conventional pneumatic drive system.

With reference to FIG. 2, the central rod 11 has a central duct 17 thatis connected at the upper end to a compressed air source (not shown) andis closed at the lower end by a spring-loaded valve 18 returned to theclosure position by a spring 19.

In FIG. 2, the accumulator 1 is shown associated with the other workstation, which is an injection station 16. The manufacturing cycle for ahollow body starts with this station, as will now be explained.

At the injection station 16, a screw injection machine of known type isused for bringing a thermoplastic resin into a malleable state and forinjecting it into the accumulation chamber 12. FIG. 2 shows only an endportion of the injection nozzle 20, which fits against the outer casing3 of the accumulator 1. A predetermined quantity of resin 35 is thusinjected into the accumulator 1 so as to fill the accumulation chamber12. To bring the resin 35 to the optimum temperature for thebiaxial-orientation molding phase and to maintain it thereat, thetemperature in the accumulation chamber 12 is regulated by means of anelectrical resistor 21 and the circulation of a fluid in the circuit ofthe inner jacket 8.

Thus, the accumulator 1 serves both for precise metering of the quantityof resin needed to obtain a given hollow body and for precise thermalconditioning of the material to be molded. This accumulator, with twothermal conditioning surfaces, allows any type of plastic to beconverted within very broad thermoplastic and thermoelastic temperatureranges. In addition, the material is conditioned in the accumulatorwithout maintaining the internal tension due to injection. Finally, thethermal conditioning in the accumulator of the material to be moldedhelps to prevent sink marks due to the material cooling prematurely.

On the basis of this situation, the operation of the machine will beexplained with the aid of the diagram shown in FIG. 8, in which eachhorizontal step represents a time step of about 0.5 s.

At step 22, the accumulator 1 is moved by the support turntable to thebiaxial-orientation molding station 2, which can be seen in FIGS. 1 and3 to 5. A cover (not shown) closes off the opening 6 during thismovement. In FIG. 1, the material contained in the accumulation chamber12 is not shown.

The biaxial-orientation/molding station 2 comprises an extrusion die 25,fixed to a stationary support plate 26, and a blowing mold 24 consistingof two separate shells 24 a and 24 b. The shells 24 a and 24 b areactuated in a transverse movement by a conventional mechanism foropening and closing the mold 24. The mold 24 contains an internal cavity36 that has a constriction 37 of diameter equal to the diameter of theorifice 28 of the extrusion die 25. Step 23, which starts at the sametime as step 22, represents the closure movement of the mold 24. Sincethis movement is known, the mold 24 is shown in the closed position inall the figures. Step 27 represents the locking of the support turntableto the station 2. The rim 5 is then positioned so as to fit against theupper surface of the extrusion die 25, the accumulator 1 being placedalong the axis of the extrusion orifice 28. Step 29 represents theopening of the cover that was closing off the opening 6.

Several operations then start almost simultaneously: step 30 representsthe displacement of the extrusion ram 14 for pushing the resin out ofthe accumulation chamber 12 through the opening 6; step 32 representsthe displacement of the parts of the central core 7; step 33 representsthe preblowing with a slight air pressure through the duct 17; and step34 represents the transfer of material through the extrusion orifice 28.

More precisely, in step 32, the central rod 11 is firstly moved, whichengages through the extrusion die 25 in the mold 24, being coated with auniform layer of resin 38. The central rod 11 advances at a speed twicethe speed with which the resin 35 is output through the extrusionorifice 28, thereby axially stretching the layer of resin 38 andintroducing a corresponding molecular orientation thereof. An endportion of the central rod 11 has a helical groove 39 over itsperipheral surface, which impresses a corresponding helical rib on theinner surface of the layer of resin 38, as may be seen in FIG. 3. Theslightly air-retarded preblowing through the duct 17 in the rod 11detaches the layer of resin 38 from the rod 11, after a certain axialdisplacement thereof beyond the constriction 37, preventing the resinfrom being cooled too rapidly. The layer of resin 38 detached from therod 11 is shown in FIG. 4, in which the helical rib 40 is also shown.During the preblowing step, the layer of resin 38 does not come intocontact with the peripheral wall of the cavity 36.

Behindhand on the central rod 11, the sizing sleeve 10 is also movedtoward the extrusion orifice 28. The sizing sleeve 10 penetrates the gapbetween the rod 11 and the peripheral wall of the extrusion orifice 28.The sizing sleeve 10 has an external thread 41, better seen in FIG. 2,which impresses a corresponding thread onto the inner surface of thelayer of resin 38. The sizing sleeve 10 moves down to the constriction37 of the mold 24, so as to form an internal thread 67 in the neck ofthe hollow body during manufacture. For example, the ratio of theinternal radius of the extrusion orifice 28 to the gap is about 10.

While the rod 11 is completing its movement down to the bottom wall 42of the internal cavity 36, the ram 14 and the inner sleeve 8 move untiltouching the rim 5 in order to completely empty the accumulation chamber12. Finally, the compacting sleeve 9 slides tightly between the sizingsleeve 10 and the peripheral wall of the extrusion orifice 28 down tothe lower end of the extrusion orifice 28, so as to completely expel theresin from the extrusion die 25 and to compress the material in theinterstice between the sizing sleeve 10 and the constriction 37. Theend-of-travel position of the various parts at the end step 32 is shownin FIG. 5.

After this situation, the blowing step 43 is carried out with a high airpressure, which makes the layer of resin 38 expand transversely until itcomes into contact with the walls of the internal cavity 36 and thuscompletes the biaxial molecular orientation of the material and theformation of a hollow body 50. For example, the blow ratio, that is tosay the ratio of the diameter of the extruded parison to the diameter ofthe hollow body 50, is about 3/4. Simultaneously, step 44, of returningthe extrusion ram 14 into the retracted position, and then step 45, ofreturning the parts of the central core 7 into the retracted position,are carried out. Thus, the parison is supported until it has beenfinalized. In step 45, the sizing sleeve 10 is rotated so as to unscrewits external thread 41 from the corresponding thread formed on the innersurface of the layer of resin 38. To do this, the central rod 11 iscoupled to a numerical-control rotary electric motor and the sizingsleeve 10 is coupled to the central rod 11 via a unidirectional pawldrive 66, which allows the sizing sleeve 10 to be driven in theunscrewing direction and also allows the sizing sleeve 10 to rotate morequickly than the central rod 11, and this prevents force being appliedon the molded thread during retraction of the sizing sleeve 10.

Step 46 represents the closure of the cover for closing off the opening6. Step 47 represents the cooling of the hollow body 50 down to theglass transition temperature of the material and below it. Step 48represents the corresponding plastication of the hollow body 50. Next,step 49 represents the opening movement of the mold 24 for ejecting thefinished hollow body 50. Step 51 represents the unlocking of theturntable and step 52 the movement of the turntable in order to bringthe accumulator 1 back to the injection station 16.

Preferably, several identical accumulators will be provided in a knownmanner, these working simultaneously, in parallel, at the variousstations. In this case, step 53 represents an initialization step of themodule for controlling the molding machine so as to start a new cyclewith another prefilled accumulator 1. As can be seen in FIG. 8, the workcycle at the station 2 lasts about 15 s. Steps 52 and 23 b are in fact arepetition of steps 22 and 23 that inaugurates this new cycle, whichwill be carried out in an identical manner to that just described.

The hollow body 50 obtained by the method just described has a uniformwall thickness, a helical rib 40 over its inner surface, which increasesits pressure resistance, and an internal thread in its neck. Other formsof ribs may be obtained in a similar manner, by modifying the path ofthe groove or grooves on the central rod 11. For example, a plurality ofparallel peripheral annular grooves produces a plurality of parallelannular ribs in the hollow body 50, and parallel axial grooves produceaxial ribs in the hollow body 50.

In step 32, the ratio of the speed of the central rod 11 to the outputspeed of the resin 35 through the extrusion orifice 28 controls theaxial stretch ratio of the layer of resin 38 and can be chosen accordingto the desired properties. This stretch ratio is equal to 2 in theexample described above.

A second embodiment of the manufacturing method according to theinvention and a corresponding variant of the molding machine will now bedescribed with reference to FIG. 6. The same reference numerals are usedto denote identical or similar elements to those of the firstembodiment.

As can be seen in FIG. 6, in the blowing mold 24 the internal cavity 36has a shoulder face 54 at right angles to the wall of the constriction37. FIG. 6 also shows annular ducts 55 for the circulation of aheat-transfer fluid in the extrusion die 25 and in the constriction 37,so as to regulate the temperature of the resin in these regions.

During the blowing step, since pressure injection takes place throughthat end of the central rod 11 lying at the bottom of the mold 24, thelayer of resin 38 is pressed against the walls of the cavity 36 from thebottom of the mold upward. The right-hand half of FIG. 6 shows the layerof resin 38 substantially as obtained during the blowing step 43 in thefirst embodiment. In the second embodiment, the sizing sleeve 10 and thecompacting sleeve 9 both continue to move toward the interior of themold 24 during the blowing step. Thus, a flat 56 on the layer of resin38, which is adjacent to an end portion 58 attached to the sizing sleeve10, is driven a certain distance away from the shoulder face 54 and isthus folded toward a lower portion 57 of the layer of resin 38, which isattached to the peripheral wall of the cavity 36. The flat 56 remainsmore flexible than the remainder of the layer of resin 38 since theabsence of contact with the mold 24 and the coating mandrel slows downthe rate at which it cools.

The left-hand half of FIG. 6 shows the flat, with the reference 56 a,approximately in its position when the sleeves 9 and 10 reach the end oftheir travel. In this embodiment, the compacting sleeve 9 also sweepsover the constriction 37 of the blowing mold 24 and the threaded part ofthe sizing sleeve 10 enters the main cavity of the mold 24. Finally, theblowing operation is completed with higher pressure, which turns downthe folded flap against the end portion 58, as shown by the reference 56b, forming a right-angle bend of material. A neck with a double wall andan internal thread is thus obtained. The rest of the method is identicalto the first embodiment.

The hollow bodies obtained by the methods described above may have manyapplications, for example for water treatment, for filtration or for thepackaging of chemical, food, pharmaceutical or cosmetic products. Hollowbodies of large capacity, for example 200 liters, may be manufactured.In particular, it is possible to manufacture hollow bodies resistant tohigh internal pressures, because of the quality of their walls and thepresence of reinforcing ribs on their inner surface, for example anaerosol bomb body designed to withstand a pressure of 30 to 35 bar. Thewall thickness is controlled by the width of the gap existing around thecentral rod 11 in the extrusion orifice 28.

FIG. 7 shows an alternative embodiment of the central rod 11, in whichit has two portions 11 a and 11 b having a smaller diameter than therest of the rod 11, in order to form, by coating, a parison with astepped thickness, and thus to obtain a hollow body having a steppedperipheral wall as regards its thickness and/or its diameter. Thethinned portions 11 a and 11 b are thus used to obtain a greater wallthickness at the bottom and top of the hollow body 50, these beingregions where the greatest pressure is exerted when the hollow body isemployed as a pressurized container.

EXAMPLE

FIG. 9 shows a hollow body 60 obtained by means of the device accordingto the second embodiment described and used as a container for aportable fire extinguisher 61. The hollow body 60 is manufactured from apolymer resin crosslinked by ionic bonds, this resin being known by thebrand name Surlyn and manufactured by DuPont®. This material exhibitsexcellent transparency, good scratch resistance, a wide processingtemperature range and very good resistance to organic solvents. The wall62 has a substantially uniform thickness e that varies between 3 and 5mm, in order to withstand a pressure of 55 bar. Its inner surface has ahelical rib 63. The neck 64 of the hollow body 60 has a double wall andan internal thread for an expulsion device 65 to be screwed onto it.

1. A method of manufacturing a biaxially oriented hollow body (50) bythe extrusion blow molding of a thermoplastic resin, comprising thesteps consisting in: bringing said resin (35) into a malleable state;filling an accumulator (1) with a predetermined quantity of said resin,said accumulator comprising an accumulation chamber (12) defined betweena central core (7) and an outer wall (3) having an end opening (6);regulating the temperature of the resin in said accumulator from bothfaces of the accumulation chamber; placing (22) a blowing mold (24) withan internal cavity (36) having an open constriction (37) incommunication with said end opening; axially displacing (32) a moveablemandrel (10, 11) from said central core through said accumulationchamber, said end opening and said constriction of the internal cavity,so as to coat a portion of the mandrel projecting from said central corewith a layer of resin (38); continuing (32) the axial displacement ofthe moveable mandrel in said internal cavity (36) at least as far as anintermediate level between said open constriction and an opposite endwall (42) of said internal cavity, simultaneously pushing (30) saidpredetermined quantity of resin out of said accumulation chamber with anoutput speed that is lower than the displacement speed of the mandrel,in order to stretch said layer of resin (38) axially; applying (43)fluid pressure to said inner surface of the layer of resin through saidmandrel in order to stretch said layer of resin transversely as far asthe walls of said internal cavity and obtain a biaxially oriented hollowbody (50) having a neck corresponding to the constriction of theinternal cavity; and letting said hollow body cool (47) down to a rigidstate, retracting (45) said mandrel and ejecting (49) said hollow bodyfrom the blowing mold.
 2. The method as claimed in claim 1,characterized by the step consisting in impressing an external relief(39, 41) of the projecting portion of said mandrel (11) onto an innersurface of said layer of resin (38) so as to obtain a hollow body havinga corresponding internal relief (40, 67).
 3. The method as claimed inclaim 2, characterized in that said external relief includes at leastone threaded mandrel portion (10) for obtaining a corresponding thread(67) on the inner surface of said hollow body.
 4. The method as claimedin claim 3, characterized in that said moveable mandrel comprises aperipheral sleeve (10) that constitutes said threaded mandrel portionand a central rod (11) that can slide axially relative to saidperipheral sleeve, and in that, in the displacement step of the mandrel(32), said peripheral sleeve (10) is brought into said constriction (37)of the internal cavity so as to clamp said layer of resin (38) betweensaid threaded mandrel portion and a wall of said constriction, and inthat, in the retraction step (45) of the mandrel, said peripheral sleeve(10) is made to undergo an axial rotational movement so as to disengagesaid peripheral sleeve from the internal thread (67) obtained in aconstriction of the hollow body.
 5. The method as claimed in claim 4,characterized by the steps consisting in: displacing the peripheralsleeve (10 from the constriction (37) toward the interior of theinternal cavity (36) during the blowing step (43), so as to fold a flat(56) of said layer of resin (38) between a portion (57) of said layer ofresin pressed against the wall of the internal cavity of the blowingmold and an end portion (58) of said layer of resin fastened to theperipheral sleeve; and pressing said folded flat (56 a, 56 b) againstsaid end portion (58) of said layer of resin fastened to the peripheralsleeve at the end of the blowing step.
 6. The method as claimed in claim1, characterized in that said end opening (6) of the accumulator andsaid constriction (37) of the blowing mold communicate through anextrusion orifice (28) of an extrusion die (25), and in that acompacting sleeve (9) is moved around said mandrel (10, 11) in saidextrusion orifice, said compacting sleeve being inserted between saidmandrel and a wall of said extrusion orifice (28) so as to completelyremove the resin from the extrusion orifice in the internal cavity (36)of the blowing mold.
 7. The method as claimed in claim 1, characterizedin that, in the displacement step (32) of the moveable mandrel, theaccumulation chamber (12) is completely emptied through the extrusionorifice (28).
 8. The method as claimed in 4 claim 1, characterized inthat said mandrel (11) is displaced substantially as far as the end wall(42) of the internal cavity.
 9. A device for implementing the method asclaimed in claim 1, comprising: a resin accumulator (1) that includes anouter wall (3) and a central core (7) that define, between them, anaccumulation chamber (12) capable of receiving a predetermined quantityof thermoplastic resin (35) in a malleable state, an end opening (6)made through said outer wall, an extrusion ram (14) arranged so as toslide between said outer wall and said central core in order to expelsaid predetermined quantity of resin from said accumulation chamberthrough said end opening; a biaxial-orientation blowing mold (24) withan internal cavity (36) having an open constriction (37) that can beplaced opposite said end opening and an end wall (42) on the oppositeside from said open constriction; a mandrel (10, 11) that can moveaxially between a retracted position inside said central core (7) andprojection positions, in which positions a portion of said mandrelprojecting from said central core is engaged through said end openingand said constriction of the internal cavity, said mandrel having anaxial internal duct (17) opening onto the outside of said mandrel atsaid projecting portion and a valve (18, 19) for selectively opening andclosing said internal duct; controlled drive means for selectivelydisplacing, so as to slide axially, said extrusion ram and said mandrel;and a pressure source connected to said internal duct of the mandrel,characterized in that the outer wall (3) of the accumulator is providedwith a heating means (21), in that the central core (7) of theaccumulator is provided with an internal circuit (13) intended for thecirculation of a heat-transfer fluid and in that said mandrel (11) canmove in said internal cavity (36) at least as far as an intermediatelevel between said constriction (37) and said end wall (42).
 10. Thedevice as claimed in claim 9, characterized in that said mandrel (11)has at least one groove (39) for obtaining a rib (40) of correspondingshape on the inner surface of said hollow body (50).
 11. The device asclaimed in claim 10, characterized in that said groove or each of saidgrooves (39) follows a closed annular path or a substantially linearaxial path or a helical path.
 12. The device as claimed in claim 9,characterized in that said mandrel includes at least one threadedmandrel portion (10) for obtaining a corresponding thread on the innersurface of said hollow body.
 13. The device as claimed in claim 12,characterized in that said moveable mandrel comprises a peripheralsleeve (10) that constitutes said threaded mandrel portion and a centralrod (11) that can slide axially relative to said peripheral sleeve, andin that said drive means are capable of axially displacing said centralrod and said peripheral sleeve so as not to be in phase and of rotatingat least said peripheral sleeve in the unscrewing direction of thethread of the peripheral sleeve.
 14. The device as claimed in claim 13,characterized in that the central rod is rotated axially by said drivemeans, a unidirectional coupler being placed between said peripheralsleeve and said central rod in order to rotationally couple saidperipheral sleeve to said central rod in said unscrewing direction andto rotationally decouple said peripheral sleeve from said central rod inthe opposite direction.
 15. The device as claimed in claim 9,characterized in that said end opening (6) of the accumulator and saidconstriction (37) of the biaxial-orientation blowing mold communicatethrough an extrusion orifice (28) of an extrusion die (25), a compactingsleeve (9) being placed around said mandrel (10) and able to moveaxially between a retracted position in said central core (7) of theaccumulator and a deployed position, in which position said compactingsleeve is inserted between said mandrel and a wall of said extrusionorifice so as to completely remove the resin from the extrusion orificein the internal cavity (36) of the biaxial-orientation blowing mold.